A wireless power transmission or a wireless energy transfer refers to a technology of wirelessly transferring electric energy to desired devices. In the 1800's, an electric motor or a transformer employing the principle of electromagnetic induction has been extensively used and then a method for transmitting electrical energy by irradiating electromagnetic waves, such as radio waves or lasers, has been suggested. Actually, electrical toothbrushes or electrical razors, which are frequently used in daily life, are charged based on the principle of electromagnetic induction. Until now, the long-distance transmission using the magnetic induction, the resonance and the short-wavelength radio frequency has been used as the wireless energy transfer scheme.
Recently, among wireless power transmitting technologies, an energy transmitting scheme employing resonance or an energy transmitting scheme, in which a transmission side employs resonance and a reception side employs magnetic induction, has been widely used.
The wireless power transmitting scheme may easily charge electronic appliances such as a portable device by wirelessly transferring power through coils which are installed in a transmission side and a reception side, respectively.
In the wireless power transmitting scheme, the coupling state of the coils between the transmission side and the reception side may vary according to a power transmitting state and the like.
However, according to the related art, since an impedance of a load side connected to the reception side is fixed, power transmission efficiency may be reduced as the coupling state varies.